A class of astronomical objects that appear on optical photographs as starlike but have large redshifts quite unlike those of stars. They were first observed in 1961 when it was found that strong radio emission was emanating from many of these starlike bodies. Over 600 such objects are now known and their redshifts can be as high as 4. The redshifts are characteristic of the expansion of the universe. This cosmological redshift is the explanation of the high observed redshifts of quasars favoured by most astronomers. (A few, however, maintained that the redshift could be a local Doppler effect, characteristic of movement relative to the earth and sun of nearby objects in the Galaxy, or a gravitational effect.) If the redshifts are cosmological, quasars are the most distant objects in the universe, some being up to 10¹⁰ light-years away. The exact nature of quasars is unknown but it is believed that they are the nuclei of galaxies in which there is violent activity. The luminosity of the nucleus is so much greater than that of the rest of the galaxy that the source appears pointlike. It has been proposed that the power source in a quasar is a supermassive black hole accreting material from the stars and gas in the surrounding galaxy. There is a considerable amount of evidence in support of this point of view.

The name quasar is a contraction of quasistellar object (QSO) or quasistellar galaxy (QSG). Quasars that are also radio sources are sometimes called quasistellar radio sources (QSS).